This invention is directed to an improved digital display driving circuit for driving digital display cells, and in particular, to a digital display driving circuit for only effecting AC driving of liquid crystal display cells in order to avoid deterioration thereof.
Due to the minimum current requirement of liquid crystal display cells, such display cells are particularly advantageous for use in miniaturized, battery-operated electronic measuring, testing and calculating instruments, such as pocket and desk type calculators and electronic wristwatches. Nevertheless, there are two characteristics of liquid crystals that must be taken into account, when utilizing same in battery-operated electronic instruments.
First, in order to avoid rapid deterioration of the liquid crystals, the liquid crystal display cells must be driven by an alternating current, hereinafter referred to as "AC drive". Specifically, the voltage across the respective electrodes defining the liquid crystal display cell must be energized in such manner as to reverse the potential across the liquid crystals during each successive energizing thereof. To this end, liquid crystal display driving circuits include interfacing circuits adapted to receive an AC driving signal and a data signal for selecting the display cells to be energized. However, if the AC driving signal ceases to be applied to the interfacing circuit, the selected liquid crystal display cells continue to be driven by the direct current applied to the interfacing circuit to energize same, thereby effecting a rapid deterioration of the liquid crystals.
Secondly, the voltage produced by the DC battery is insufficient to effect driving of liquid crystal display cells, thereby requiring a booster circuit for elevating the voltage applied to the display drive interface circuit. Usually, such booster circuits receive an AC signal produced by the same source utilized to generate the AC signal applied to the interface circuit to effect AC drive of the display cells. Because such AC signals are usually produced by an oscillator circuit energized by the DC supply, when the DC supply voltage begins to drop, an insufficient voltage is provided for effecting oscillation of the oscillator circuit producing the AC drive signals. Accordingly, the AC drive signals are no longer applied to the booster circuit and/or interface circuit, thereby causing the display cells to be driven by the unboosted direct current voltage produced by the DC supply. In such event, the contrast in the energized display cell is less than completely satisfactory and, as detailed above, the liquid crystals rapidly deteriorate. Accordingly, circuitry for preventing a DC drive current from being applied to the liquid crystal display cells is desired.